The present invention relates generally to the production of glass sheets and, more particularly, to an improved method of and apparatus for tempering glass sheets to specific standards in a mass production operation.
In many countries, there are official regulations specifying the manner of fragmentation of tempered glass sheets for use as windows of motor vehicles. Such regulations commonly require that the fractured glass sheet should not contain dangerously elongated particles. For example, British Standard BS 5282 basically prohibits the presence of particles longer than 60 mm in which the length is not less than four times the width. British Standard BS 5282 specifies that the number of particles included in any 50 mm.times.50 mm square traced on the glass sheet should be within a limited range and further specifies a maximum permissible area of each particle.
In the recent automobile industry history, there has been a trend to reduce the thickness of the glass of windows to thereby aid in the overall reduction of the gross weight of the vehicle. However, for glass sheets of 3.5 mm or less in thickness, it becomes difficult to temper the glass sheet so as to meet the official regulations for fragmentation, and the difficulty is further compounded when the glass sheet is of relatively large size particularly with respect to the exclusion of elongated particles (spline).
A recent European safety code, ECE R43, Category I, has set new standards for tempered glass. According to the specification, glass 3.5 mm and thinner falls into category I, with a thickness tolerance of plus or minus 0.2 mm. There are two critical requirements of the code for glass in this category. First, there is a fragmentation range of 40 particles minimum and 450 particles maximum in a area of 50 mm.times.50 mm. Second, elongation fragments (i.e. spline) can not exceed 75 mm in length. The advent of these more tightly controlled safety specifications for tempered glass has presented increased challenges to the glass manufacturer.
U.S. Pat. No. 4,662,926 discloses a method for toughening a glass sheet by heating it to a temperature above the strain point and then quenching the heated glass sheet by blowing a cooling medium against both sides of the glass sheet from two opposed sets of nozzles. The cooling medium is caused to impinge on each side of the glass sheet in a concentric circular pattern such that the glass sheet is more effectively quenched and consequently more highly toughened in a generally circular central region.
Another process that has been successful in producing bent, tempered sheets of glass, such as are commonly used in glazing closures for automobiles and the like, is the horizontal press bending technique. This technique generally includes heating pre-trimmed flat sheets of glass to their softening or bending temperatures by advancing them on a roll conveyor through a heating furnace, bending the heated sheets to a desired curvature or shape between a pair of complementary mold members and then tempering the same by chilling the bent sheets in a controlled manner to a temperature below the annealing range of glass.